The rigors of designing a portable heart rate monitor (HRM) are enough to give anyone a case of angina. For starters, cardiac monitors must meet the highest standards for safety, reliability, and accuracy.

Designers must also contend with the power constraints of button cell batteries. Add the market demand for added functionality but no increase in space, power, or cost to the requirement list and the heartburn sets in.

Fortunately, there’s relief. Using a variety of the latest micropower, high-precision IC components, it’s possible to design a low power heart rate monitor (HRM) that also packs in additional features.

The most critical function of the low-power ICs is extending the battery life of an HRM, which measures a patient’s heart rate in real time or records it for later study. Portable HRMs operate from batteries for long periods of time, and require low current consumption. Low-voltage batteries have been used for decades as the single power source in Holter monitors and other portable ECG systems to ensure safety (the last thing a heart patient or the sensitive equipment needs is a zap of “hot” line voltage). Micropower ICs operate on low voltage and current, thus conserving battery power.

This article looks at the objectives and circuitry of HRM front ends (see Figure above), including lead off detection, the voltage reference, front-end instrumentation amplifier (in amp), passive-component sizing, signal processing in the microconverter, waveforms, applicable standards, accuracy, and power consumption.

David Guo is an applications engineer at Analog Devices, Inc. in the Precision Signal Conditioning Group. He is responsible for integrated amplifier products such as the AD8236 micropower in-amp. He can be reached at david.guo@analog.com

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A good design with about 0.66 ma or 660 micro amps power consumption and can be easily marketed.The analog front end details are given by the author and the link to the micro converters are not available.